The invention relates to a printhead suitable for use with image forming systems, and more particularly relates to an arrangement of electrode and dielectric layers within a printhead for optimizing print quality and performance.
There are many different printing technologies utilized today for creating and reproducing images in an image forming system. Several of these technologies include a general process of charging a surface of a latent image receiving member, such as a drum, with a latent charge image. The term drum illustrates a common structure for support of the latent image-receiving member. The drum can also be one of several other architectures including a curved latent image receiving member, or a flexible dielectric belt, which moves along a predetermined path. A drum can also be an imaging member, such as a liquid crystal, phosphor screen, or similar display panel in which the latent charge image results in a visible image. The drum typically includes on an exterior surface thereof, a material that lends itself to receiving the latent charge image, such as a dielectric layer. Accordingly, the term drum used herein shall mean all such structures or devices.
A number of organic and inorganic materials are suitable for the dielectric layer of the drum. The suitable materials include glass enamel, anodized, flame or plasma sprayed high-density aluminum oxide, and plastic, including polyamides, nylons, and other tough thermoplastic or thermoset resins, among other materials.
The drum rotates past an image-forming device, such as a printhead, which produces a stream of accelerated electrons as primary charge carriers. The electrons reach the drum, landing in the form of a latent charge image. The latent charge image then receives a developer material to develop the image. The image is applied to a medium, e.g., a sheet of paper, by press or electrostatic transfer to form a printed document.
The printhead is most often a multi-electrode structure that defines an array of charge generating sites. Each of the charge generating sites, when the electrodes are actuated, generates and directs toward the drum a stream of charge carriers, e.g., electrons, to form a pointwise accumulation of charge on the drum that constitutes the latent image. A representative printhead generally includes a first collection of drive electrodes, e.g., RF-line electrodes, oriented in a first direction across the printing direction. A second collection of control electrodes, e.g., finger electrodes, oriented transversely to the drive electrodes, forms cross points or intersections with the drive electrodes constituting an array of charge generating sites at which charges originate. A dielectric layer couples to, and physically and electrically separates and insulates, the RF-line electrodes from the finger electrodes.
The printhead can also include a third electrode structure, often identified as a screen electrode. This screen electrode couples to the finger electrodes by an insulating structure, such as a spacer layer. The screen electrodes have a plurality of passages aligned with the charge generating sites, to allow the stream of charge carriers to pass through. The screen electrode can be a single conductive sheet having an aperture aligned over each charge generating site. Polarity of charge carriers passing through the passages, or apertures, depends on the voltage difference applied to the finger and screen electrodes. Polarity of particles accumulated on the drum to create latent image is determined by the voltage difference between the screen electrode and the drum surface. The charged particles of appropriate polarity are inhibited from passing through the aperture, depending upon the sign of their charge, so that the printhead emits either positive or negative charge carriers, depending on its electrode operating potentials.
One issue associated with current printing technology is that there is a significant size variation in dots landing on the drum. For example, conventional printheads have typically from 12 to 20 RF-line electrodes. Charge carriers that are generated from the outermost RF-lines deposit on the cylindrical drum in the form of a dot that is relatively smaller than those dots resulting from charge carriers emitted from more centrally located RF-line electrodes. This is because of a difference in distance between the outermost RF-lines and the curved surface of the drum, and the innermost RF-lines and the curved surface of the drum. Charge carriers emitted from the outermost RF-lines travel in a weaker electric field and must overcome greater distance to reach the drum surface than charge carriers emitted from the innermost RF-lines. The variation in the travel conditions causes this anomaly. The minimum air gap, and therefore the maximum electric field in between the screen and the dielectric drum, is normally directly beneath the more central RF-line electrodes. With decreasing drum diameters, the variations become increasingly severe because the curvature of the drum surface increases.
To compensate for the dot size variations, some prior art solutions have included enlarging the charge emitting sites, i.e., screen holes, or increasing the number of cycles incorporated in a single RF burst. Such compensation methods are unique to a particular drum/printhead combination, and do not compensate for blooming effects.
Blooming is essentially spreading the charged particles around the targeted area. Such spreading is a result of repulsive electrostatic forces between arriving and already deposited charge particles. The level of blooming depends on a ratio of these repulsive forces and attractive forces created by the electric field in the printhead/drum region. The resulting blooming effect has a substantial impact on dot geometry.
The surface charging effect also slightly deflects dots, which are aimed nearby. If charge dot is to be deposited in the proximity of one or more charged dots that have already been laid down, the interaction between the particle beam and previously deposited charge results in the dot lateral shift. Because the printing order of dots is constant, similar conditions and dot quality repeat in each printed line. Therefore, all deviations are organized in the process direction, which reveals itself as streaks of different intensity of print. This effect is known as Venetian blinding.
Therefore, charge density profiles of the dot latent images still depend on the screen hole positions. Further, the respective differences vary with charge level. Such issues significantly deteriorate the print quality of the printhead for grayscale or color images.
For the foregoing reasons, there exists in the art a need for a universal printhead that functions independent of the diameter of the charge receiving dielectric drum, while concomitantly optimizing print quality and lessening the effects of blooming or Venetian blinding (print quality descriptors that are well known in the art). The present invention is directed toward further solutions in this art.
In accordance with one aspect of the present invention, a printhead is provided having a first layer of electrodes covered and sealed by a dielectric material. Further layered upon the dielectric material is a second layer of electrodes. Each of the electrodes from the first layer intersects with each of the electrodes from the second layer and forms a plurality of charge generating sites. The charge generating sites are generally disposed in only two rows.
In accordance with another aspect of the present invention, the first plurality of electrodes includes two elongate electrode RF-lines. The second plurality of electrodes includes a plurality of finger electrodes that are arranged in a plurality of rows. Each finger electrode is coupled to a separate contact pad.
In accordance with still another aspect of the present invention, the first plurality of electrodes includes two rows of RF-line electrodes that are broken into sections or segments. The second plurality of electrodes includes a plurality of finger electrodes that are arranged in a plurality of rows. There is a single contact pad coupled to each of a subset of the second plurality of electrodes.
In accordance with yet another aspect of the present invention, the first plurality of electrodes includes a plurality of collector electrodes that are coupled to relatively shorter segments of said plurality of finger electrodes. The second plurality of electrodes includes a plurality of finger electrodes. Pairs formed from the plurality of electrodes are coupled to a single contact pad.